The invention relates to shaped articles containing balsa wood and method of producing them.
Balsa wood is a type of wood which is very easy and simple to process. Apart from being used to build floats and as a replacement for cork, balsa wood is used by model builders for aircraft and ship models. However, balsa wood has the greatest significance as the core material of composite materials in a sandwich construction method, for example in boat, ship and yacht construction, in aviation, as in sailplane and small aircraft construction, in space travel and as the core or core material of rotor blades of wind power installations, for example. The good insulating properties of balsa wood are also used for insulation against heat and cold, for example for fuel tanks. In the technical application area, the low volume weight and the unusually high compressive strength parallel to the grain direction in relation to the low gross weight are utilised.
A so-called centre layer material is produced for the applications mentioned. The basic component produced for this is the so-called cross-cut wood panel. For this purpose, balsa boards machined on four sides, also called squared wood or balsa squares are glued to form large blocks, for example with a cross-section of about 600×1,200 mm and then sawed transverse to the grain direction to form panels of any thickness, for example about 5 to 50 mm, and then sanded to the precise thickness size. This light cross-cut wood panel can absorb very strong compressive forces over the area, but is intrinsically very unstable. Highly loadable composite materials are obtained, for example, by the application, on one or both sides, transverse to the grain direction, of plastics material panels, plastics material panels or layers reinforced with glass, plastic or carbon fibres, metal panels or sheets, wood panels, veneers, woven fabrics, foils etc. onto the centre layer material or a cross-cut wood panel.
To construct highly curved components, such as, for example, in the production of hulls for boats or sailing yachts, a thin fibre which is nonwoven, knitted fabric or woven fabric glued to the cross-cut wood panel on one side and the cross-cut wood panel is scored from the opposite side in a right parallelepiped or cuboid shape through to a thin web. The panel which is prepared in this manner can be made into any concave or convex shape and can be adapted to a curved shape, such as that of a boat or float or a spherical tank.
Balsa wood is a natural product. Therefore, the properties of the balsa wood may change within the wood of one harvest or even in sections from a trunk of one tree. In relation to this, for example, the bulk density, the shrinkage, the compressive strength, the tensile strength etc., and the pore content may vary. Defective locations in the trunks, such as internal cracks, so-called red heartwood or water core, grain tangles or mineral specks, if not removed in time with loss of wood, can influence the regularity of the properties of a cross-cut wood panel.
As a balsa wood trunk is round, but the cross-cut wood panel to be produced therefrom is produced from a large number of rectangular boards, the trunk has to be sawn in the grain direction or course of the grain, and transversely thereto. The sawn boards are tightly stacked, pressed by means of the mutual contact faces and glued and then sawn again transverse to the grain direction. By peeling off the tree bark, sawing off the rounded parts by a chordal or tangential cut and sawing into panels or boards, only about 25% of the available wood is utilised for technical use. The remainder accumulates as chips, cuttings and sawdust.